A differential assembly is a gear system typically employed in rear-wheel drive motor vehicles to transfer power from the propeller shaft to the output axle shafts. The differential assembly uses a drive pinion gear which mates with an adjoining ring gear to act as a conduit for the power transfer from the propeller shaft to the output axle shafts. More particularly, the power is transferred from the propeller shaft to the differential case, wherein the output axles shafts are splined to the side output gears at a right angle to the longitudinal axis of the propeller shaft.
The drive pinion gear is also a component of the driveline assembly of the motor vehicle. In most rear-wheel drive motor vehicles, the driveline assembly includes a propeller shaft and a drive pinion gear. Functionally, in broad terms, the driveline assembly connects the transmission to the side output axles. In overview, the driveline is the conduit which transmits engine power to the driving wheels.
In operation, as the propeller shaft rotates, it turns the drive pinion gear to which it is splined or otherwise attached. The drive pinion gear rotates the ring gear and in turn the differential gear case attached to the ring gear. The differential pinion gears mounted within the differential case interact with the differential side output gears that are splined to the output axle shafts.
Further during operation, when the motor vehicle is moving straight, the ring gear and differential case rotate in concert. In this straight movement, the differential pinion gears do not turn on their own axes, but apply equal effort to each of the side output gears and their attached output axle shafts. On the other hand, during vehicle turning movement, the resistance against the rotation of one output axle shaft increases as the inner and outer wheels turn at different speeds. This difference in speed encourages the differential pinion gears to rotate on their own axes and turn the side output gear on the axle bearing the resistance.
A differential case is relatively heavy and rigid to withstand the torque transmitted through the differential case which causes it be subjected to torsional loads. Such rigidity of the differential case assures interaction of the differential pinion gears and side output gears, which are attached to the differential case.
With respect to some configurations of a vehicle's driveline, a universal joint is connected to the input shaft of the drive pinion gear by way of a yoke welded to each end of the propeller shaft. This arrangement of power transfer components ideally serves to compensate for any changes in the driveline. However, with such an arrangement, the power transfer may not be as efficient as possible between the propeller shaft and the differential.
Moreover, such an arrangement in this adjoining area between the differential and driveline assemblies also serves to increase the size and thus the weight of the assemblies, as well as the entire vehicle. Specifically, the more components of the present arrangement, the greater the vehicle's "unsprung weight". "Unsprung weight" refers to the vehicle body componentry which is not supported by springs, including the wheels, differential assembly and axles. Conversely, "sprung weight" involves the use of the springs to suspend the vehicle frame, body, engine and powertrain above the wheels. Because the smoothness of a vehicle's ride increases as its unsprung weight decreases, a low unsprung weight is often a goal of vehicle manufacturers and their suppliers.
Further, the more components that make up the differential and driveline assemblies, the higher the cost of the assemblies in terms of labor and piece price. Additionally, the increased number of components which form the differential and driveline adjoining area may result in more frequent maintenance.
The prior art has attempted to address weight reduction in the differential assembly. For example, U.S. Pat. No. 4,723,464 issued to Welschof et al discloses a differential assembly having constant velocity joints integrated with the side output gears. However, this reference does not disclose improving the efficiency, weight, size, cost and overall lifetime with regard to the area of the adjoining differential gear and driveline assemblies.
Consequently, there is a need for an improved differential and/or driveline assembly which has reduced weight characteristics (i.e. unsprung weight) while maintaining the stability of the component, thereby reducing costs and leading to a smoother riding vehicle. The assembly should also serve to transfer power more easily and efficiently from the driveline to the differential. Such an improved assembly should further allow for a lower weight and less expensive design which is easier to assemble, thereby leading to labor cost savings. The need has also developed for a differential-driveline assembly having increased life and longer durability, resulting in reduced maintenance. The improved differential-driveline assembly should also accommodate any reasonable torque, bending angle, and speed subjected upon it by the propeller shaft.